Recent developments in the microelectronics industry have allowed commercial integrated circuit (IC) chip manufacturers to achieve a very high packing density within a single IC chip. Submicron features can now be produced on a regular basis. Although the IC industry has gone through revolutionary changes in packing density and device performance, the electronics packaging industry has not seen the same degree of size reduction. One reason for this difference lies in the need to use discrete passive and active electronic devices on circuit boards as well as electrical interconnections to obtain fully functioning IC devices. Since each of the discrete devices must be placed onto the circuit board and bonded in place, various physical constraints dictate the size that the circuit board must maintain.
A variety of methods have been developed for depositing layers of materials onto each other. One method used for depositing metal layers onto other metal substrates is known as laser cladding. In this process, a metallic substrate is used as a deposition surface. A laser is then used to create a molten puddle on the surface of the deposition substrate and the cladding material is fed into the molten puddle in either wire or powder form. The cladding material is consumed in the molten metal puddle and forms the cladding layer. In this fashion, a wear-resistant surface can be applied to a ductile material or an object can be built through sequential layer deposition methods. Due to the relatively high heat input and localized heating of laser cladding processes, the cladding operation is primarily limited to more ductile metallic materials. When this process is applied to materials that are sensitive to thermal shock, catastrophic failure of the deposited material or substrate materials generally occurs.
U.S. Pat. No. 4,323,756 discusses a method similar to cladding for depositing layers of materials onto each other. This method produces rapidly-solidified bulk articles from metallic feedstock using an energy beam as a heat source to fuse the feedstock onto a substrate. Repeated layers are deposited in order to arrive at a three-dimensional finished product. However, the use of a laser to melt the substrate creates excessive heat in the part, causing distortion and residual stress within the part being made. Also, the high energy level required of a laser suitable for this method causes inefficiencies throughout the system.
Another method used for depositing materials is known as the thermal spray process. This process also deposits new material onto a substrate. The materials to be deposited are melted and sprayed onto the deposition surface in droplet form. The deposition material can be supplied in either powder- or wire-form, and is fed into a heated region to be melted. As the materials are melted, a gas stream causes the materials to be directed at the deposition surface at some velocity. The gas can serve to aid in the formation of the droplets. These droplets then form a large diverging jet of molten material that can be used to coat a large area of a particular substrate. One of the limitations of the thermal spray process is in its lack of ability to produce fine features, such as those produced by laser cladding processes. However, there are also advantages provided by the thermal spray process. Since there is little substrate heating, residual stress within the deposited layers is not as significant as that which occurs during the laser cladding process. In addition, as the molten particles solidify they are still spreading out due to the kinetic energy of the particle. This energy can, in effect, serve to counter the residual stress in the part since the energy due to spreading will be in the opposite direction as that due to residual stress. Due to the reduced residual stress, which occurs during the thermal spray process, a much broader range of materials can be deposited. This includes depositing ceramics, plastics, metals and carbides onto dissimilar material surfaces.
The use of nozzles in thermal/plasma spray processes has added certain advantages to these processes; however, the disadvantage of inability to produce fine features remains. U.S. Pat. No. 5,043,548 describes a laser plasma spraying nozzle and method that permits high deposition rates and efficiencies of finely divided particles of a wide range of feed materials. This system uses powdered materials that are carried to the interaction regions via a carrier gas and lasers to melt these particles. However, this system relies solely on the use of a laser created plasma to melt the particles before they are ever introduced to the deposition region. In fact, the carrier gas is often a mixture which promotes ionization and, as such, the formation of a plasma. The formation of a plasma results in melting of the powder particles before they ever come into contact with the deposition substrate. In addition, the beam is diverging such that when it does impact the deposition substrate, the beam irradiance is sufficiently low so that no melting of the deposition substrate occurs. A great distance between the focal point of the laser and the central portion of the plasma is maintained to prevent the substrate from melting. This distance, ranging from 1-6 inches, is a characteristic of this method. The materials are deposited in either a liquid or gaseous state. This design provides a unique method for coating parts; however, it has never been intended for fabrication of multi-layered parts. Due to the diverging nature of the powder material, this plasma technique fails to provide the feature definition necessary for fabricating complex, net-shaped objects.
The laser spraying process is yet another method for depositing layers of materials onto each other. U. S. Pat. No. 4,947,463 describes a laser spraying process in which a feedstock material is fed into a single focused laser beam that is transverse to a gas flow. The gas flow is used to propel the molten particulate material towards the surface onto which the spray deposition process is to occur. In this patent, use of a focused laser beam to create a high energy density zone is described. Feedstock material is supplied to the high-energy density zone in the form of powder or wire and carrier gas blows across the beam/material interaction zone to direct the molten material towards the surface onto which the spray process is to deposit a film. One critical point of the '463 patent is that it requires the high energy density zone created by the converging laser to be substantially cylindrical. Realizing that efficient melting of the feedstock material is related to the interaction time between the focused laser beam and the feedstock material, '463 also describes projecting the feedstock material through the beam/material interaction zone at an angle off-normal to the beam optical axis. This provides a longer time for the material to be within the beam and increases the absorbed energy. Also, this method primarily controls the width of the deposition by varying the diameter of the carrier gas stream, which provides variation on the order of millimeters. Although this resolution is adequate for large area deposition, it is inadequate for precision deposition applications.
U. S. Pat. No. 5,208,431 describes a method for producing objects by laser spraying and an apparatus for conducting the method. This method requires the use of a very high-powered laser source (i.e., 30 to 50 kW) such that instantaneous melting of the material passed through the beam can occur. The high laser power levels required by '431 a necessary because the laser beam employed in the process is not focused. As such, a very high-powered laser source is required. In fact, this process is essentially limited to CO.sub.2 and CO lasers since these lasers are the only sources currently available which can generate these power levels. These lasers are very expensive and, as a result, limit application of this method.
The spray processes provide another approach to applying a broad range of materials to substrates of similar or dissimilar composition in order to create thin films of material. However, there exists a need for improved geometric confinement of the materials streams in order to provide a technology platform on which to build a means to directly fabricate interconnected active and passive electronic components onto a single substrate, thereby achieving an integrated solution for electronic packaging.